The present invention relates to a hybrid former and specifically to an inlet into a twin-wire zone in a hybrid former for a paper machine. The former comprises a lower-wire loop in which there is a single-wire initial portion of the forming zone and draining elements arranged inside the lower-wire loop. After the draining elements, inside an upper-wire loop and the lower-wire loop, which together form the twin-wire zone, wire-guide, forming and draining elements are arranged. The former includes an upper-wire unit in which there is the upper wire which is guided by rolls and which is guided by the breast roll onto a pulp layer formed on the single-wire initial portion of the lower wire. In the former, in the twin-wire zone thereof, there is a dewatering and forming unit or units. The present invention also relates to a method in a hybrid former of a paper machine.
A gap former is mostly better than a hybrid former both in view of the quality of the paper and in view of the runnability, but for many paper mills, a gap former, which also requires modernization of the existing headbox, is an excessively costly construction. In a number of cases, a more advantageous construction would be an upper-wire unit placed close to the headbox, by means of which construction at least a part of the favorable properties of a gap former are obtained. In modernizations of paper machines, this would permit the use of the existing fourdrinier headbox.
It is a general object of the present invention to develop a novel inlet solution for the twin-wire zone of a hybrid former in particular for modernizations of existing fourdrinier wire parts and formers marketed by the assignee under the trade mark SYM FORMER.TM. in view of improving the paper quality, widening the range of use, and/or increasing the running speeds of paper machines. It should, however, be emphasized that the former in accordance with the present invention is also suitable for use in completely new formers.
In web formers of paper machines, a number of different forming members are used. The primary function of these members is to produce compression pressure and pressure pulsation in the fiber layer that is being formed. By means of the pressure and pulsation, the draining of water out of the web that is being formed is promoted while the formation of the web is improved. The forming members include various forming shoes which are usually provided with a curved ribbed deck and over which the forming wires placed one above the other and the web placed between them are curved. In the area of these forming shoes, water is drained through the wire placed at the side of the outside curve by the effect of its tensioning pressure, and this draining is aided further by a field of centrifugal force. Draining of water also takes place through the wire placed at the side of the inside curve, which draining is typically intensified by means of a vacuum present in the chamber of the forming shoe. The ribbed deck of the forming shoe produces pressure pulsation which both promotes the dewatering and improves the formation of the web.
Also, in the prior art, so-called MB-units are known, through which two opposite wires run. Inside one of the wire loops in the prior art MB-units, there is loading equipment, and inside the other, opposite wire loop, dewatering equipment is arranged which is provided with a set of guide and dewatering ribs in opposed relationship to the loading equipment. As known in the prior art, the MB-unit is usually placed on a fourdrinier portion so that the MB-unit is preceded by a single-wire portion of considerable length in which a substantial proportion of draining takes place before the web runs through the MB-unit. With respect to the details of construction of the prior art MB-units, reference is made, by way of example, to the assignee's Finnish Patent Application Nos. 884109 and 885607 (corresponding to U.S. Pat. Nos. 5,185,004 and 4,988,408, respectively, the specifications of which are hereby incorporated by reference herein).
In the prior art, a number of different hybrid and gap formers are known which are provided with an MB-unit or units as referred to above. With respect to these formers, reference is made to the following Finnish Patent Applications: 884109, 885608, 904489, 905447, 920228, 920863, 924289, 930927, 931950, 931951, 931952, 932265, 932793 and 934999. FI 885608, FI 932265 and FI 932793 correspond to U.S. patent application Ser. Nos. 07/442,013, 08/246,176 and 08/262,138, respectively, the specifications of which are hereby incorporated by reference herein. FI 904489 and FI 920228 correspond to U.S. Pat. Nos. 5,215,628 and 5,395,484, respectively, the specifications of which are hereby incorporated by reference herein.
The inlet of the twin-wire forming zone has proved a critical point in general and in particular when MB-forming units are used. It has been noticed that the initial part of the MB-unit has a substantial effect, e.g., on the retention and on the porosity of the paper. Problems are produced in particular because, when the upper wire enters into contact with the top face of the pulp web that is being formed, the fiber structure "freezes", in which case any unevenness present in the upper wire or in the top face of the pulp layer is seen as flaws in the finished paper. The unevenness is more likely to occur when the upper wire is brought into contact with the top face of the pulp layer in a curved area while the upper wire runs unsupported at that location.
With respect to the prior art closely related to the present invention, reference is made to the assignee's Finnish Patent Application No. 934999 (filed Nov. 12, 1993) which describes a former in which at an initial portion of the twin-wire forming zone, there is a revolving alignment and forming roll arranged inside the lower-wire loop. The alignment and forming roll is in tangential contact with the lower wire or curves the twin-wire zone at a small angle a, which angle is selected in the range of about 0.degree. to about 5.degree.. The alignment and forming roll is substantially immediately followed by the draining and forming unit which comprises sets of ribs and in whose area, water is drained primarily through the upper wire while aided by the negative pressures in the draining chamber or chambers in the draining and forming unit.
In the former described in FI 934999 and in other, corresponding formers, it is a drawback that the forming roll placed at the beginning of the twin-wire zone inside one of the wire loops produces such a high dewatering pressure that, in particular with thicker paper grades, the structure of the web is broken, i.e. the web is "crushed". An obvious solution for this problem is to increase the diameter of the forming roll to about 2 meters which, however, makes the construction quite costly and spacious. In view of the web formation and the symmetry of draining, it would be of great importance that, in hybrid formers, the twin-wire forming zone can be started in an area in which the dry solids content of the web is from about 0.7% to about 1.7% and in which the face of the fiber layer that is placed against the lower wire has already been couched to a suitable extent, but the top face is still almost at the headbox consistency. However, with earlier former constructions, it has for the most part been possible to start the twin-wire zone only at a location at which the dry solids content is about 2.5% and the thickness of the web layer is maximally from about 5 mm to about 6 mm. It has not been possible to achieve even a consistency as low as this (2.5%) with thicker paper grades.
Shifting the upper-wire unit quite close to the headbox involves prior art in itself known. These prior art constructions involve certain problems and drawbacks, for which problems the present invention offers novel solutions. When a covered former roll is used at the inlet of a hybrid former, a sufficient support and a stable run are obtained for the wires but, at the same time, the inlet consistency of the pulp web is confined to a range that is not optimal in view of the quality of the paper. On the other hand, the use of dewatering ribs and of the pulsating draining pressure produced by them in the inlet area is problematic in gap formers, as is well known, for example, in view of formation of streaks and in view of retention.
With respect to the additional prior art related to the present invention, reference is made to the Finnish Patent Application No. 913480 assigned to Valmet Tampella Oy, in which a gap accomplished by means of an open roll and having no covering angle is used together with a curved ribbed deck following after the gap.